Systems of this kind are known in the art, e.g., from U.S. Pat. No. 4,043,065.
In prior art systems, the emergency brake piston monitors the existence of emergency braking upon every pressure drop in the brake line, by means of a control chamber exhaust valve. Thus, whenever there is a pressure drop in the brake line, the emergency brake piston assists in determining whether service braking or emergency braking is involved.
If service braking is involved, the control chamber (QA chamber) is connected to atmosphere via an exhaust valve, until a predetermined pressure relationship between the brake line pressure and the control chamber pressure is restored at the emergency brake piston. If emergency braking is involved, one control chamber located at one side of the control piston of the high pressure exhaust valve device is also connected to atmosphere, so that the control piston is controlled by the quick action pressure in the other control piston chamber, in order to move the high pressure valve and the brake line emergency exhaust valve to open position.
To perform this task, the emergency brake piston requires a certain sensitivity of reaction to pressure changes in the brake line. In the case of emergency braking, it must cut through quickly enough to produce rapid enough switching of the high pressure exhaust valve device.
Recently, pulsating brake accelerators have been in demand for brake control valves. These repeatedly bleed air pulses from the brake line to atmosphere during each stage of braking. It has proven especially advantageous to actuate such brake accelerators with air from the quick action chamber, which air then flows out to atmosphere. For this purpose, the control chamber is connected to the brake accelerator by a brake accelerator inlet nozzle which is in series with an inlet valve switchd to open position upon even slight pressure drops in the brake line on the basis of a low pressure level.
For control of such a brake accelerator inlet valve, it is known to supplement the emergency brake piston with a separate control piston which, like the emergency control piston, is acted upon by the brake line pressure on one side and the quick action pressure on the other side, and which reacts with sufficient sensitivity to the pressure differences between these. U.S. Pat. Nos. 4,070,068 and 4,206,949 may be referred to in this connection.
In order to minimize the costs of construction, it has proven advantageous to control the brake acceleration inlet valve by means of the emergency brake piston, so as to eliminate the separate control piston. This, however, requires the emergency brake piston to react with sufficient sensitivity to pressure differences between the brake line and the control chamber, and also to cut through quickly enough, upon emergency braking, to effect emergency braking.
U.S. Pat. No. 3,716,276 discloses an emergency brake piston which not only monitors emergency braking but also controls the air for the control of a pulsating brake accelerator. To accomplish this double task, the emergency brake piston controls a gate valve. However, in this type of application, gate valves are inferior to seated valves in various respects.
An emergency brake piston which controls a control chamber exhaust valve on the one hand and a brake accelerator inlet valve on the other hand is known from U.S. Pat. Nos. 4,226,482 and 4,139,239. In these, the brake accelerator inlet valve is already in the form of a seated valve on one side of the emergency brake piston, and the control chamber exhaust valve on the other side of the emergency brake valve is in the form of a gate valve. A disadvantage is that the brake accelerator inlet valve is connected to the brake line, so that the pulsating brake accelerator is actuated by the brake line air escaping to atmosphere through a brake accelerator inlet nozzle. It is further disadvantageous to configure the control chamber exhaust valve as a gate valve. Apart from these disadvantages, the disclosed emergency brake piston, which is acted upon by the brake line pressure on one side and the control chamber pressure on the other side, is not sensitive enough to both control the brake accelerator inlet valve and to sense in sufficient time the existence of emergency braking.
An emergency brake piston which accomplishes the double task of controlling both the control chamber exhaust valve and the brake accelerator inlet valve is disclosed in U.S. patent application No. 492,318. Both valves are in the form of seated valves, and the brake accelerator inlet valve is inserted in a connecting line to the control chamber.
A further simplification of the total construction and maintenance cost of brake control valve systems of the type discussed hereinabove could be achieved if the service brake piston and the emergency brake piston were of substantially the same effective diameter. Since the diameter of the service brake piston must have a certain minimum size due to its function, this size being substantially greater than the functionally required size of the emergency brake piston, equal sizing of the two pistons would cause the latter to be too large, adversely effecting the need for maximum switching speed during emergency braking.
Finally, the construction costs of known brake control valve systems could be further simplified if it were possible to eliminate the valve, previously required, which during emergency braking switches so as to vent the control chamber through a nozzle, with the objective of assuring that the high pressure exhaust valve system is returned by spring force to its original position within a predetermined time. Such a valve, which is required in prior art brake control valve systems, could be eliminated if the valve in question could be so arranged between such selected spaces that it comes into play only during emergency braking, without a supplemental valve.
Incidentally, the high pressure emergency exhaust valve system of U.S. Pat. No. 4,043,605 has the drawback that the high pressure valve is in the form of a gate valve with O-rings, and that, for rapid cutting through of the valve system during emergency braking, the air of the control chamber must be substantially vented from the piston chamber on one side of the control piston which switches the valve system. To enable the most rapid possible venting of the said piston chamber, which assures sufficiently rapid switching of the high pressure exhaust valve system, the above-noted patent requires a supplemental and expensive gate valve arrangement controlled by the emergency brake piston, in order to bleed additional air from the piston chamber to atmosphere during emergency braking.